You can find the following formulae (or variations thereof) in most statistics
textbooks, especially descriptive statistics dealing with probability. Sample Size ...
Sample Size Formulas You can find the following formulae (or variations thereof) in most statistics textbooks, especially descriptive statistics dealing with probability. Sample Size - Infinite Population (where the population is greater than 50,000) SS
=
Z2 x (p) x (1 – p) _____________ C2
SS = Sample Size Z = Z-valueA (e.g., 1.96 for a 95 percent confidence level) P = Percentage of population picking a choice, expressed as decimalB C = Confidence interval, expressed as decimal (e.g., .04 = +/- 4 percentage points) A Z-values (Cumulative Normal Probability Table) represent the probability that a sample will fall within a certain distribution. The Z-values for confidence levels are: 1.645 = 90 percent confidence level 1.96 = 95 percent confidence level 2.576 = 99 percent confidence level Example: SS = 3.8416 x .5 x .5 .0016 SS = 600 Sample Size – Finite Population (where the population is less than 50,000) SS New SS = _________________ ( 1 + ( SS – 1 )) Pop Pop = Population (e.g., 4,300) Note: Calculate the sample size using the infinite population formula first. Then use the sample size derived from that calculation to calculate a sample size for a finite population.
